How do I set variables?

[Sketch]

Simple question, but elusive to me. How would I define a new variable to be used in a template.

Say I wanted to use $aaron and have a new bit that would display "Aaron is AwesomE!" wherever I put that in my templates.

In a similar vein, what if I wanted to use some PHP if/else to determine if a variable is set and if it is display it and if it is not, do not. An example of this would be in the postbit of my forum http://forums.livetheworship.com where I added $post[field7] (CD currently being played) to my postbit. I was trying to figure out how to test to see if a user set anything and if they did, display that variable and if they did not, then it would not display at all. Since I couldn't figure it out I had to settle on using (nothing) in my table for field7. Any ideas how to do these things.

In Summary:
1. how Do I add a new bit and then use it in the template system?
2. How do I set a variable in PHP and then use it in the templates?

Thanks.
Aaron

PS. If I can figure these simple things out, whole new worlds of mids will open up to me.